A catalytic reformer apparatus is used, for example, in fuel cell power plant systems to produce a hydrogen fuel gas from a hydrocarbon fuel for use in a fuel cell. One method of producing the hydrogen is through steam reforming. In steam reforming, steam and the hydrocarbon fuel are passed over catalyst beds which are disposed in reformer tubes.
Because the reaction is endothermic, the reformer tubes are disposed within a burner gas region which includes burner cavity where fuel is burned to provide heat to the reformer tubes. The tubes are exposed to and receive heat from heated gases and from walls bounding the burner cavity.
One example of such a construction is shown in U.S. Pat. No. 4,098,587 issued to Krar et al. which is commonly assigned to the assignee of this application and was previously assigned to a predecessor in interest of the present assignee. As shown in FIG. 3 of Krar, the reformer tube is provided with a shield which is disposed about the reformer tube to prevent the tubes from receiving excessive radiant heat from the gases in the burner cavity and from the walls of the cavity. This more evenly distributes heat among and around all of the reformer tubes permitting the reformer tubes to be closely packed within the cavity and reducing temperature differences between reformer tubes.
The sleeves in Krar may be made from either a low thermally conductive material (insulating material), such as a ceramic, or thermally conductive material, such as stainless steel. Krar notes that while the best shielding is provided by low thermally conductive material, thermally conducting material can also provide good shielding. The sleeve may fully or only partially surround the tube and may have slots or other openings and cutouts which control the flow of burner cavity gases around the tubes.
As shown in FIG. 3 of Krar, the shield fits over the top of the reformer tube and surrounds the upper length of that portion of the reformer tube which is disposed in the hottest environment of the reformer. The opening in the side of the shield achieves a more uniform, circumferential temperature distribution about the reactor and permits some hot gas flow between the shield and the reformer to provide the necessary heat for the reformer.
With state of the art reformers being operated at higher and higher temperatures, cracking occasionally occurs in such shields leading to the possibility of sections falling out and resulting in local overheating and tube failure even when fibrous or castable ceramic materials are used for the shield. Accordingly, scientists and engineers are seeking to develop radiation shields having satisfactory durability even at elevated temperatures.